Suggesting a DC-DC Buck Converter for Compensating Shaft Induced Voltage and Bearing Current R. Kazemi Golkhandan Faculty of Electrical Engineering, K. N. Toosi University of Technology, Tehran-Iran reza_kazemi@ee.kntu.ac.ir M. Tavakoli Bina Faculty of Electrical Engineering, K. N. Toosi University of Technology Tehran-Iran tavakoli@kntu.ac.ir M. A. Golkar Faculty of Electrical Engineering, K. N. Toosi University of Technology Tehran-Iran golkar@kntu.ac.ir M. Jokar MAPNA E&C Co. (MECO) Karaj,Iran jokar-m@ mapnaec.com Abstract—This paper proposes a complete distributed model for investigating both the induced shaft voltage and bearing current in turbo generators due to the interaction with their static excitation systems. This affects the insulations gradually, leading to a possible electrical discharge current when the shaft voltage exceeds the dielectric breakdown voltage level of the lubricating grease film in journal bearings. Furthermore, a buck converter is proposed to overcome the stated issue, where simulations confirm the effectiveness of the proposals. Keywords-Bearing current; Buck converter; Motion control; Shaft voltage; Static excitation system I. INTRODUCTION Shaft voltages have become a serious problem in large turbo generators. There are four potential sources of shaft voltages in rotating machinery: Magnetic dissymmetry, axial shaft flux, electrostatic charge and external voltages supplied to the rotor windings [1]-[4]. The mechanism of occurrence and transmission of the first three types of shaft voltages are relatively well known and are not discussed here. External voltages supplied to the rotor windings are primarily related to the electrical machines excitation system [1]. Static excitation systems are source of shaft voltages with considerable magnitudes. The output voltage of the rectifier, however, contains harmonics in addition to the desired DC-voltage [5]. When shaft voltage with respect to the frame exceeds the dielectric breakdown of thin lubricating grease in two metal bearings on the exciter end (EE) or turbine end (TE) of generator, an electrical discharge machining (EDM) current flows through the bearings [6]. By occurrence of the dielectric breakdown, a high current impulse is created. These current pulses result in the appearance of pits and transverse flutes burnt into the bearing race [7]. In order to predict the problems related to shaft voltages and bearing currents, developing a circuit model of the system is of special importance. Equivalent circuit models have been proposed for investigation of shaft voltage. In [8], a model is proposed to observe the existence of significant shaft voltages induced by the PWM voltage source inverters in motors. Amman et al. in [3] have proposed a circuit model to investigate shaft voltage in a large turbo generator. Each coil in the circuit was lumped and modeled by one inductance and two capacitances and then these circuit models were connected in series with each other. The proposed model in [3] represents transmission from excitation winding to the shaft line in the frequency range of 50Hz to 1MHz. This paper deals with the shaft voltages arising from static excitation systems. A complete distributed circuit model of parasitic couplings between adjacent windings, between windings and rotor shaft and also between windings and the stator is proposed. The aim of this approach is investigating shaft voltage and bearing current in a typical 200MVA Ansaldo turbo generator. In this procedure, in addition to the characteristics of the proposed models in [3] and [8], each turn is modeled individually, skin effect of conductors in high frequencies and also the value of the parasitic capacitances between each turn and its adjacent turns are calculated and finally, the equivalent circuits are connected in series with each other and in parallel with the shaft line and bearing equivalent circuit models. In order to reduce shaft voltage and bearing current, some countermeasures have been investigated and compared in this paper. Among the solutions, one of them is described in detail and applied to the proposed model. Simulation results of the proposed model verify the existence of the shaft voltage; also, when a buck converter is applied as the compensator, then the bearing voltage is lowered down to a desirable level. II. SHAFT VOLTAGE ANALYSIS A. Model Of the System A scheme of the system to be investigated is depicted in Figure 1. The elements are as follows: 1. Excitation transformer 2. Rectifier (static excitation system) 3. Excitation winding 4. Journal bearings 5. Shaft line 2011 IEEE Symposium on Industrial Electronics and Applications (ISIEA2011), September 25-28, 2011, Langkawi, Malaysia 978-1-4577-1417-7/11/$26.00 ©2011 IEEE 19